This invention relates generally to article counting apparatus and more particularly to sensing and data processing apparatus for the counting of a plurality of substantially identical thickness objects tightly stacked adjacent to one another. More specifically, this invention relates to improvements with respect to the article counting apparatus disclosed by S.P. Willits, et al, in U.S. Pat. No. RE 27,869, William L. Mohan, et al, in U.S. Pat. No. 4,373,135, William L. Mohan, et al, in Pat. No. 4,542,470, and William L. Mohan, et al in U.S. Pat. No. 3,813,523, hereinafter the Willits, Mohan 1, Mohan 2 and Mohan 3 patents, respectively.
While the foregoing prior art devices generated satisfactory counting data for stacked objects in most instances they were particularly designed to define the very minute contrast areas between the adjacent stacked objects each of which has basically identical reflectivity associated with the edges of the several stacked objects. The Willits reference disclosed the concept of a pair of sensors "pitch-matched" to the object edge thickness to resolve the difficulty encountered with such material in generating a non-ambiguous signal where there is essentially no brightness gradient between the adjacent objects but there is an increasing brightness gradient from sheet-to-sheet. Willits describes the differential summing of the outputs of a pair of pitch matched sensors to provide an approximation of the first derivative of brightness across the elements comprising the stack. This system works well yielding unambiguous data provided the brightness slope generated by the summed sensor outputs continues in either a positive or negative direction or stays constant.
If, however, the brightness gradient alternates from positive to negative from sheet to sheet, the output wave train data reverts to a sub-harmonic of the desired output count frequency and, as a result, the output data becomes ambiguous. This condition arises when the objects are very tightly stacked and successive object edges appear alternately light and dark.